1. Field of the Invention
The present invention relates to a semiconductor device. In particular, the present invention relates to a semiconductor device which transmits/receives a signal to/from outside, a so-called IC chip (also called an ID chip, an IC tag, an ID tag, an RF tag, a wireless tag, an electronic tag, or a transponder) for RFID (Radio Frequency Identification).
Note that a semiconductor device here means that any device which can function by using a semiconductor property.
2. Description of the Related Art
With development of computer technologies and improvement of image recognition technologies, information identification utilizing media such as bar codes has spread widely and been used for identification of product data and the like. It is expected that the amount of information to be identified will further increase in the future. On the other hand, information identification utilizing bar codes is disadvantageous in that a bar code reader is required to be in contact with bar codes, and the amount of data stored in bar codes cannot be increased so much. Therefore, contactless information identification and increase in the storage capacity of media are required.
In view of the foregoing requirements, a contactless IC chip for RFID (hereinafter referred to as an IC chip) and a reader/writer device (also called an interrogator: hereinafter referred to as a reader/writer) have been developed. The IC chip has a memory circuit to store necessary information, and the information inside is read by a reader/writer using a contactless means, generally a wireless means. It is expected that practical application of an information processing device for reading information stored in such an IC chip will allow commercial distribution and the like to be simplified and made cheaper while ensuring high security.
In recent years, IC chips which can transmit/receive data without contact have become widely used in various fields in which automatic identification is required, such as management of valuable securities and merchandise. A card equipped with such an IC chip reads and writes data from/to an external device without contact, via an antenna with a shape adapted to the frequency band that is used when transmitting/receiving data.
A semiconductor device such as an IC chip is different from a semiconductor device which reads two-dimensional informational such as bar codes. A reader/writer is provided with an anti-collision function (also called collision avoidance function or merely anti-collision), so that the semiconductor device can read a signal from a plurality of IC chips (e.g., the ALOHA method which complies with the ISO 15693-3 standard). In addition, there is a method for performing an anti-collision function by providing a plurality of circuits in an IC chip. Reference 1 (Japanese Published Patent Application No. 2004-248310) discloses a data processing method in which a plurality of capacitors for resonance is provided and a resonance frequency value is kept constant, so that a function compatible with anti-collision between IC chips can be performed reliably.